The present invention relates to a sinterable carbon powder for the production of carbon bodies with a high density and strength without the use of binders, and also to a method of producing this carbon powder. In a further aspect, the present invention relates to a method of producing shaped carbon bodies without the use of a binder, and products obtained thereby.
The production of carbon bodies from calcined cokes with pitch binders by means of pressing, carbonization and optionally graphitizing is known. High densities are achieved if the coke is finely ground before being mixed with the binder. During the carbonization process, pores are produced in the carbon body by means of pyrolysis and distillation losses of the binder; the pores are reduced by further impregnating and carbonization processes. It is thus possible to produce graphite bodies with a density of up to 1.78 g/cm.sup.3. Since the coke and the binder carbon exhibit different properties, stresses arise during the carbonizing and the graphitizing due to different shrinkage. These stresses result in defects on the boundary surfaces. For this reason, carbon bodies produced in this manner exhibit a relatively low strength.
Therefore carbon powders have been sought which exhibit suitable binding and sintering properties and which can thus be processed to carbon bodies without additional binders by means of pressing, carbonizing and optionally graphitizing.
EP-A-1,156,051 and EP-B-1,157,560 disclose carbon powders with a content of more than 95% by weight benzene-insoluble and more than 10% by weight quinoline-insoluble components which still contain 4 to 15% by weight evaporable components. Carbon bodies with a density of 1.85 to 2.05 g/cm.sup.3 can be produced with these carbon powders at appropriately high compacting pressures by means of graphitizing. The carbon powder should still contain 2.5 to 15% by weight quinoline-soluble but benzene-insoluble components (.beta. resins).
Such a product is produced by means of the thermal treatment of coal-tar pitch at 350.degree. to 600.degree. C., solvent extraction of the pitch (in order to obtain a benzene-insoluble residue), and a new thermal treatment of the residue under inert gas at 250.degree. to 500.degree. C. Pitch mesophases are produced in the isotropic pitch matrix during the first thermal treatment. A carbon powder consisting of mesophases with a coating of .beta. resins remains after the second thermal treatment by means of the extracting of the benzene-soluble components of this matrix. Therefore this process also involves a two-component system with non-optimal properties of strength. A further disadvantage is the fact that the .beta. resin coating is not resistant to ageing, therefore the cohesion of the sinterable powder decreases with storage.
GB 1,492,832 suggests treating a pitch obtained by distillation, polymerization or condensation with an atomic H/C ratio of up to 0.8 with oxygen, sulfur or halogens. Compounds containing the latter can also be used. The treatment is carried out until a certain amount of these elements has reacted with the pitch. This is determined to be a function of its H/C ratio. The untreated pitch is at first pre-comminuted in order to be able to carry out the reaction in a simpler manner, and is subsequently ground down to a particle size of a maximum of 10 .mu.m. Carbon bodies with a high density and strength are produced from this powder. In addition to the H/C ratio, the oxygen content and the coking residue, the content of quinoline-insoluble components is also indicated in the examples, which is approximately in a range of 7 to 95% by weight. The content of toluene-insoluble or .beta. resins is as little indicated as in the start of softening and therefore obviously plays no part in the invention.
The next development in the prior art is represented by EP-A-0,283,211. A pitch with a mesophase content of at least 50% by volume, preferably more than 75% by volume, especially more than 95% by volume, is ground to a grain fineness of less than 10 .mu.m and oxidized in order to obtain a sinterable carbon powder. The oxidation is carried out, as in GB 1,492,832, as a function of the H/C ratio of the mesophase pitch. The grinding of the pitch to a particle diameter of less than 10 .mu.m prior to the oxidation makes it possible to produce carbon bodies which exhibit approximately twice as high a bending strength as comparable carbon bodies (according to GB 1,492,832) under conditions which are otherwise the same.
The grinding fineness necessary for both methods is preferably achieved by wet grinding in a suitable solvent. A drying of the carbon powder is therefore necessary before the oxidation, which is quite expensive in the case of powders which are so fine, if no solvents are to be entrained in the subsequent process steps. A further disadvantage of the improved carbon powder according to EP-A-0,282,211 is the fact that very high pressures are required for producing dense and bend-resistant carbon bodies. Densities of more than 1.6 g/cm.sup.3 in the carbonized bodies are only achieved at pressures of approximately 500 MPa. The bending strength of these bodies is then approximately 130 to 140 MPa. Such high compacting pressures can be realized into the interior of the powder charges only with difficulty in industrial processes, especially in the production of large formed bodies.
There was therefore the problem in the art of developing a sinterable carbon powder and a method of its production in which the cited disadvantages are avoided, so that superior carbon bodies can be produced at an industrially justifiable expense.